Semiconductor chips typically are thin and flat, with relatively large front and rear surfaces and small edge surfaces. The chips have contacts on their front surfaces. Typically, chips are provided as packaged chips having terminals suitable for connection to an external circuit. Packaged chips typically are also in the form of flat bodies. Ordinarily, the packaged chips are arranged in an array on a surface of a circuit board. The circuit board has electrical conductors, normally referred to as “traces” extending in horizontal directions parallel to the surface of the circuit board and also has contact pads or other electrically conductive elements connected to the traces. The packaged chips are mounted with their terminal-bearing faces confronting the surface of the circuit board and the terminals on each packaged chip are electrically connected to the contact pads of the circuit board.
Memory chips typically are mounted in this manner. An unpackaged memory chip typically has numerous data contacts and one or a few select contacts. The chip is arranged to ignore data or commands appearing at the data terminals unless the appropriate signals are applied to the select contact or contacts. A conventional packaged memory chip has data terminals connected to the data contacts and has select terminals connected to the select contacts. In a conventional system, numerous identical packaged memory chips can be connected in an array with the corresponding data terminals of the various packaged chips connected to common traces and with the select terminals of the various chips connected to unique conductors, so that each conductor is associated with one, and only one, chip. Data can be written onto an individual chip by supplying the data on the common traces and by applying a selection signal on the unique trace associated with the particular chip where the data is to be written. The remaining chips will ignore the data. The reverse process is employed to read data from a particular chip. Such a circuit can be built readily using the conventional horizontal chip array and using identical chip packages for all of the chips in the array.
In the conventional arrangement, the theoretical minimum area of the circuit board is equal to the aggregate areas of all of the terminal-bearing surfaces of the individual chip packages. In practice, the circuit board must be somewhat larger than this theoretical minimum. The traces on the circuit board typically have significant length and impedance so that appreciable time is required for propagation of signals along the traces. This limits the speed of operation of the circuit.
Various approaches have been proposed for alleviating these drawbacks. One such approach is to “stack” plural chips one above the other in a common package. The package itself has vertically-extending conductors that are connected to the contact pads of the circuit board. The individual chips within the package are connected to these vertically-extending conductors. Because the thickness of a chip is substantially smaller than its horizontal dimensions, the internal conductors can be shorter than the traces on a circuit board that would be required to connect the same number of chips in a conventional arrangement. Examples of stacked packages are shown, for example, in U.S. Pat. Nos. 5,861,666; 5,198,888; 4,956,694; 6,072,233;and 6,268,649. The stacked packages shown in certain embodiments of these patents are made by providing individual units, each including a single chip and a package element having unit terminals. Within each unit, the contacts of the chip are connected to the unit terminals. The units are stacked one atop the other. Unit terminals of each unit are connected to the corresponding unit terminals of other units. The connected unit terminals form vertical conductors of the stacked package, also referred to as buses.
However, providing a circuit with individual select connections in a stacked package introduces additional complexities. Because the vertical conductors extend through the terminals of the various units, the interconnections between the contacts of the chip and the unit terminals of each unit in the stack should be different in order to provide connections to unique vertical conductors. For example, in a four-chip stack having four vertical buses for carrying selection signals, the bottom unit may have a select contact of its chip connected to a unit terminal that forms part of bus number 1; the next unit may have a corresponding select contact of its chip connected to a terminal that forms bus number 2; and so on. This need for customization of the units adds complexity to the manufacturing process. For example, U.S. Pat. No. 4,956,694 describes units having chip carriers with a set of intermediate terminals in each unit. These intermediate terminals are connected to the contacts on the chip and are also connected to the terminals of the unit. The interconnections are made by wire bonds. The pattern of wire bonds differs from unit to unit. This arrangement inherently requires a relatively large chip carrier, which adds to the cost and bulk of the package. Moreover, the manufacturer must handle and stock multiple different wire bonded units. Sugano et al., U.S. Pat. No. 5,198,888, uses individualized chip carriers in the various units. These chip carriers have leads defining different interconnect patterns for the select contacts and the associated terminals. This, again, adds to the cost and complexity of the manufacturing process. U.S. Pat. Nos. 6,268,649 and 6,072,233 use customized units as well. It would be desirable to reduce the cost and complexity associated with providing customized units in a stacked package.
It would also be desirable to provide a compact stacked package and to provide a stacked package with good heat transfer from the chips within the stack to the external environment as, for example, to the circuit board or to a heat spreader overlying the top of the package. Further, it would be desirable to provide such a package using readily-available equipment and using components that can be fabricated readily.